Classical Catadioptric Telescopes employing concave spherical mirror objectives can be fast and aberration free in the visible spectrum, but suffer from field curvature and restrictions due to poor accessibility of their focal surfaces. In addition, correctors for such telescopes are either aspherical (See Schmidt, "A Rapid Coma-free Mirror System", reprinted in Amateur Telescope Making, Book Three, pp. 373-375, incorporated herein by reference.), or require tight centering and radius control along with aspherical "retouch" for best performance. (See Maksutov, "New Catadoiptric Meniscus Systems", Journal of Optical Society of America, Vol. 34, No. 5. May, 1944, pp. 270-284, incorporated herein by reference).
Spherical concave objective mirrors and spherical corrector lenses are desired for Catadioptric Telescopes because the mirrors can be economically produced in large sizes and spheres are easy to produce in unlimited quantities by well-controlled production processes and are easy to test. Therefore, spherical optics have tremendous advantage over aspheric optics such as parabolic telescope mirrors and aspheric corrector elements which always require scarce skills for their fabrication, and production of aspherics is frequently unpredictable or limited. In addition, except for field curvature, Catadioptric Telescopes employing spherical mirror objectives can be designed to provide larger fields of view than telescopes having aspheric objectives because the spherical surface has no unique optical center, and at the same time still can have good correction of aberrations. Consequently, the image of an object situated off the axis of the telescope and which would be aberrated by an aspheric primary may not be so aberrated by spherical optics.
The main problem of spherical mirrors in Catadioptric Telescopes is (negative) spherical aberration all across the field which must be corrected (nulled). Also the mirror's coma, astigmatism, and negative field curvature should be corrected for wide field use, and the introduction of false color should be avoided.
Obviously, small sub-aperture sized corrector elements, which can be located in the convergent beam should be much easier to produce than those required for telescopes employing a full aperture sized corrector. Most classical Catadioptric Telescopes require full sized corrector elements (located upstream of the objective) and still the field is not flat; because the mirror has a strongly curved (negative) focal surface and the corrector must (in order to yield positive spherical aberration) be either negative or too weak to null this curvature. Another class of Catadioptric Telescopes uses a sub-aperture sized negative achromat made of crown and flint glass or a Barlow lens having positive spherical aberration to correct to negative spherical aberration of spherical primary mirrors. This type of corrector is small and located within the convergent beam, but these telescopes are f/8 or slower and have non-negligible secondary color and tend to have only small useful fields. In addition, the negative optical power of negative achromats obviously can not correct the negative field curvature due to a concave spherical primary mirror objective and always exacerbates it.
Still another class of Catadioptric Telescopes uses a negative power, or weak, one-glass sub-aperture sized corrector and requires more than two corrector elements for best performance, and/or requires (additional) corrective element(s) downstream of an intermediate image, real or virtual, to obtain aberration correction. (See R.D. Sigler, "All-spherical Catadioptric Telescope with Small Corrector Lenses", Applied Optics, Vol. 21, No. 15, 1 Aug. 1982, pp. 2804-2808, incorporated herein by reference.) But again, these correctors all have net negative optical power or are too weak to correct the negative field curvature due to the spherical primary mirror. Also, these designs tend to be slow (i.e. f/7-f/15.7) and to have detrimentally small fields of less than one degree, or one-half degree due particularly to color and astigmatism.
These classes of classical and other Catadioptric Telescopes also require tight centering tolerances for the corrector surfaces and also for the telescope assembly. In addition, due to requirements on spacing between elements of the classes of telescopes that use sub-aperture sized corrector elements, all these have lenses awkwardly positioned along the optical axis as obstructions within the main tube of the telescope. It would be a great convenience, and remove obstructions to the optical path, for these corrector elements to be removed to the side as with a Newtonian style focus; and also sensitivity to positioning and centering errors should be substantially reduced.
Difficulties due to a curved focal surface can be accommodated by using curved photographic plates or curved films to fit the focal surface in the case of photography; or by the use of "field flatteners". Obviously, the use of curved photographic films, plates or curved photosensitive devices (photodetectors) imposes major restrictions in convenience an efficacy of use and fabrication which are avoided when a lensless standard camera or other device having a flat film plane or plano photographic plates, cut or roll film, flat photodiode array, or other such flat detector can be used directly in conjunction with a flat field telescope.
Field flatteners are extra lenses placed in contact or nearly in contact with the photographic film or surface and thus are a hindrance for visual use, and as they are quite near the image plane, dust and dirt can become quite noticeable. Alternatively, field curvature can be avoided by using a particular Cassegrain configuration in which the radius of curvature of the secondary mirror is chosen to be the same as the radius of the primary and reflects converging light back toward the primary and usually through a hole in it. This choice of equal radii for both mirrors tends to restrict the design; but much worse is the fact that all Cassegrain configurations are extremely sensitive to tilt and decenter of the secondary mirror. Cassegrains are neither simple nor easy to construct because the secondary has power (it is curved in order to extend the focus to provide accessibility to it). The Newtonian style plano secondary mirror is preferred in this regard in that it merely reflects light from the primary toward the side of the instrument for easy accessibility of the focal surface and does not amplify the focal length of the primary mirror and in in principle aberration free. Thus the Newtonian Secondary is relatively tolerant of tilt and decentering errors and also does not increase telescope focal length nor reduce its optical speed.